Pharmaceutical tablets are usually prepared by the instantaneous compression of a powder, comprising the active ingredient and an excipient, between two punches in a die. The force for compression may be supplied by either the upper punch or by both the upper and lower punches, but in neither case does all of the applied force go into compressing the powder. Although some of the force is lost in heat and sound energy a major proportion is absorbed in overcoming die wall friction. These frictional forces are sometimes sufficiently great as to prevent tablet compression altogether, and in other cases the appearance of the tablets is unacceptable; for example the tablets may be chipped, capped or laminated rendering them unsuitable for further process.
In order to obviate these problems it has been usual to incorporate a lubricant, especially magnesium stearate, in the powder or granules to be tabletted or moulded, normally in a proportion of from 0.25% to 1% by weight. Magnesium stearate has been found to be one of the most efficient tablet lubricants and it also acts as an anti-adherent, preventing powder from sticking to punch faces and die walls. Other lubricant powders, may, however be used as, for example, salts of benzoic acid and polyethylene glycols.
The use of magnesium stearate lubricant has, however, given rise to a number of problems, especially in the production of pharmaceutical tablets but also for other moulded products. The principal problems are as follows:
(a) it is an extremely hydrophobic powder which can adversely affect the bioavailability of drugs and is undesirable in soluble tablets where it produces a surface film or scum on the glass of water in which the tablet is dissolved.
(b) the mixing time used to incoporate the magnesium stearate in the other ingredients of the tablet formulation is critical and can influence the physico-mechanical properties of the tablets produced. For example, slight over-mixing is known to seriously reduce the strength of tablets and can produce capping or lamination which completely disrupts tablets.
(c) in common with other tablet lubricant powders, magnesium stearate is incorporated in the whole of the tablet mixture which results in a lubricant coat being formed around most of the granules or particles. This is inefficient since lubricant is only required at the interface between metal and particle surfaces. It is also undesirable since lubricant--excipient and lubricant--active ingredient contact produces poor bonding and seriously weakens the mechanical strength of the tablets produced.